(1) Field of the Invention
The present invention relates to a flow rate measuring system using a Karman vortex sensor. Such a flow rate measuring system can be used, for example, for measuring the flow rate of engine intake air.
(2) Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is considered to be one of the most important parameters for controlling the fuel injection, ignition timing, and the like, and an airflow meter is provided for measuring the same. One of the more common prior art airflow meters is the vane-type. This type, however, is disadvantageous in scale, response speed characteristics, and the like. Recently, Karman vortex flow rate meters (sensors) have been developed, which are advantageous in scale, response characteristics, and the like.
In a prior art Karman vortex sensor (see Japanese Unexamined Patent Publication (Kokai) Nos. 58-80524, 58-80525), a Karman vortex generator is inserted into a flow tube through which fluid flows. Fluctuation in pressure generated alternately on both sides of the Karman vortex generator is transmitted through a pair of pressure transmission paths to a vibration plate outside of the tube, and the rotational displacement of the vibration plate is photoelectrically detected, thereby obtaining the flow rate of the fluid. In this case, a sine-wave shaped electrical signal in response to the rotational displacement of the vibration plate is obtained, to thereby obtain the flow rate of the fluid.
When the flow rate of the fluid is obtained by the frequency of the above-mentioned sine-wave shaped electrical signal, it is common to additionally correct the flow rate of the fluid in accordance with the frequency, since the flow rate of the fluid is not completely proportional to the frequency of the Karman vortex signal (see Kokai 56-616). In addition, in a low flow rate region, it takes a long time to obtain an accurate flow rate due to the low frequency. Further, since the flow rate of the fluid is so weighted and averaged homogeneously over the entire frequency, the response speed characteristics are reduced, and an accuracy of correction is too low in a low flow rate region, and too high in a high flow rate region. Note that, for airflow meters of engines, a high response speed of the flow rate is necessary, and an accurate correction of the flow rate is also necessary, particularly in a low flow rate region.